Abrasive particles have long been used in numerous applications, including cutting, drilling, sawing, grinding, lapping and polishing of materials. A wide variety of abrasive particles can be used, depending on the specific application and workpiece. Typically, extraordinarily hard abrasive particles such as diamond and cubic boron nitride (cBN) are referred to as superabrasive particles.
Abrasive and superabrasive particles can be single crystal grits or polycrystalline grits. Most often, single crystal grits can be produced by nucleation and growth in the presence of a catalyst under high pressure and high temperature. For example, graphite powder and a catalyst, e.g., Fe, Co, Ni, or their alloys, can be layered or mixed and placed in a high pressure apparatus. Although a number of high pressure apparatuses are known, the two most common apparatuses are the cubic press and belt press. Cubic presses are typically cheaper and easier to operate, but also have limited reaction volumes. Similarly, belt apparatuses can provide larger reaction volumes, but tend to be more expensive and difficult to operate successfully. At sufficiently high temperatures and pressures within the stability region of diamond, graphite can dissolve in the molten catalyst and precipitate as diamond. Alternatively, graphite can be dispersed as flakes in the molten catalyst which then precipitates as diamond. Generally, a higher growth rate corresponds to more crystal defects, while a longer growth time allows for increased particle size. Similar behavior occurs during growth of cBN and other abrasive particles. For example, high pressure and high temperature growth of cBN can be realized using hexagonal boron nitride (hBN) and a catalyst such as an alkali metal nitride or alkaline earth metal nitride.
Abrasive grits can be further processed to form various products. For example, abrasive grits can be pulverized to form smaller abrasive fines, e.g., as small as about 0.1 μm. Alternatively, micron powder of superabrasive can be sintered to form larger abrasive bodies such as polycrystalline diamond (PCD) or polycrystalline cBN (PcBN). These larger PCD and PcBN compacts are often supported by a metal substrate, such as cemented tungsten carbide to reinforce their impact strength.
Costs of production per unit weight for superabrasive compacts tend to increase with increasing particle size, primarily due to the time required to grow large crystals. Conversely, the cost of production for abrasive compacts tends to decrease with increasing size. Thus, abrasive grit sizes seldom exceed about 1 mm, while abrasive compacts are generally larger than about 3 mm in diameter and are frequently up to several centimeters in size.
The cutting and material removal properties of single crystal particles and polycrystalline bodies can differ considerably. Specifically, polycrystalline bodies have randomly oriented microscopic grains corresponding to individual grains. This makes the polycrystalline bodies more impact resistant than single crystal particles which tend to fracture along cleavage planes which often results in shattering or failure of the entire single crystal particle as a useful abrasive particle. Further, as polycrystalline bodies fracture on a microscale, the fractures expose new sharp edges and help to maintain abrasive properties over a longer useful life. Additionally, polycrystalline bodies tend to have rougher surfaces than single crystals, making bonding with various tool matrices more secure.
In order to take advantage of the properties of polycrystalline bodies, a number of methods have been developed to produce polycrystalline particles or grits. Several common methods involve production of PCD or PcBN compacts which can then be crushed, milled, and/or acid leached to form smaller grits. While such processes do produce useful grits, they tend to have highly irregular shapes and broad particle size distributions. Another method includes cutting larger PCD bodies using wire electrical discharge machining or laser to cut the PCD bodies into smaller particles. However, this method tends to be expensive and time consuming, despite improved regular shapes and consistent size distributions.
As such, methods and materials for improved polycrystalline grits and methods of producing them continue to be sought.